Bus leakage resistance estimation for electrical isolation testing and diagnostics

ABSTRACT

Electrical bus isolation is detected for an electrified vehicle having a DC power source connected to positive and negative buses. The positive bus is connected to chassis ground, and a resulting first current is sensed that flows through a negative bus leakage resistance and a balanced leakage resistance. The negative bus is connected to chassis ground, and a resulting second current is sensed that flows through a negative bus leakage resistance and a balanced leakage resistance. The positive and negative bus leakage resistances are estimated in response to respective ratios of the first and second currents. An isolation value is compared to a threshold, wherein the isolation value is responsive to a voltage of the DC power source and a smaller one of the positive and negative bus leakage resistances. An atypical isolation is signaled when the isolation value is less than the threshold.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of co-pending U.S. application Ser. No.14/504,588, filed Oct. 2, 2014, which is incorporated herein byreference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Not Applicable.

BACKGROUND OF THE INVENTION

The present invention relates in general to electrified vehicles using ahigh voltage bus, and, more specifically, to accurate estimation of theeffective isolation resistance present between each high-power bus and achassis ground.

Electrified vehicles such as for electric vehicles and hybrid electricvehicles typically utilize a high voltage power bus driven by a DC powersource which may include storage and/or conversion devices such as amulti-cell battery pack or a fuel cell. The presence of high-voltagebuses creates a need for monitoring of the electrical isolation of eachbus with respect to the electrically conductive components of thevehicle chassis (ground).

Any leakage resistance present between a DC bus and chassis ground mustbe sufficiently large. In typical leakage resistance detection systems,there is an assumption that leakage resistance will be between one ofthe positive or negative DC buses and chassis ground. A typical leakagedetector circuit operates by periodically connecting one bus at a timeto chassis ground through a current-limiting resistance, and using theresulting current flow to calculate the leakage resistance between theopposite bus and ground. The battery voltage divided by the calculatedleakage resistance characterizes the electrical isolation.

The invention is based, in part, on a determination that conventionalleakage resistance detection systems based on supplying a current fromone bus through a known resistance to the leakage resistance between theground and the other bus may ignore a potential balanced component ofleakage resistance from both buses to ground that can sometimes resultin the mischaracterization of the electrical isolation because of apotential discrepancy in the derived leakage resistance values. Morespecifically, a resistance may exist between the positive bus andchassis ground as well as a resistance of equal value between thenegative bus and chassis ground. These resistances, both being equal invalue, are hereafter referred to as symmetrical or balanced leakageresistance. A resistance on one bus to chassis without a matching valueon the other bus to chassis is hereafter referred to as non-symmetricalor unbalanced leakage resistance. The additional current flow throughthe balanced leakage resistance may cause the prior art detection systemto overestimate the composite balanced and unbalanced resistance whichexists between one bus and chassis ground. Estimating this lattercomposite resistance is desirable in order to more accurately determinethe isolation.

One typical source of a balanced leakage resistance would be a hydrogenfuel cell vehicle, wherein a deionizer intended to remove ions fromwater being used as a coolant fails to maintain proper deionization. Asions build up, the conductance of the cooling water increases and theelectrical isolation between both positive and negative fuel cellelectrodes and ground is reduced. Another possible source of a balancedleakage resistance includes a symmetrical breakdown in cable insulation.

SUMMARY OF THE INVENTION

The invention recognizes the presence of both balanced and unbalancedleakage resistance that exist between the positive and negative powerbuses and chassis ground. By differentiating between the leakageresistances that are the same for the two buses versus leakageresistance that is not the same, the invention more accuratelydetermines the electrical isolation. Moreover, by separately identifyingthe balanced and unbalanced leakage resistances, detection and/orprediction of electrical or other vehicle conditions are enabled.

In one aspect of the invention, an electrified vehicle comprises apositive bus connectable to a positive output of a DC power source and anegative bus connectable to a negative output of the DC power source. Achassis ground is distributed within the vehicle. A first detectorcircuit is selectably activated to i) connect a first fixed resistancebetween the positive bus and chassis ground and ii) sense a resultingfirst current. A second detector circuit is selectably activated to i)connect a second fixed resistance between the negative bus and chassisground and ii) sense a resulting second current. A control circuitidentifies the positive and negative bus leakage resistances in responseto a nonlinear function (e.g., ratio) of both the first and secondcurrents. In one preferred embodiment, the current ratios provide acorrection factor that can be applied to conventionally derivedresistance values.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing one type of electrified vehicle towhich the present invention is applied.

FIG. 2 is a schematic diagram showing one typical bus structure withleakage resistances shown.

FIG. 3 is a schematic diagram showing test measurement circuits forcharacterizing bus leakage resistance.

FIG. 4 is a schematic diagram showing an idealized current flow that isa basis of a prior art leakage resistance measurement.

FIG. 5 is a schematic diagram showing an actual current flow during atest measurement which provides a basis for characterizing leakageresistance in the present invention.

FIG. 6 is a flowchart of one embodiment of the invention wherein anelectrical isolation is determined with an improved accuracy.

FIG. 7 is a flowchart of a further embodiment of the invention whereinbalanced and/or unbalanced leakage resistance estimates provide improvedvehicle diagnostics.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The term “electrified vehicle” as used herein includes vehicles havingan electric motor for vehicle propulsion, such as battery electricvehicles (BEV), hybrid electric vehicles (HEV), and plug-in hybridelectric vehicles (PHEV). A BEV includes an electric motor, wherein theenergy source for the motor is a battery that is re-chargeable from anexternal electric grid. In a BEV, the battery or other DC sourcesupplies energy for vehicle propulsion. A HEV includes an internalcombustion engine and an electric motor, wherein the energy source forthe engine is fuel and the energy source for the motor is a DC storageunit such as a battery. In a HEV, the engine is the main source ofenergy for vehicle propulsion with the battery providing supplementalenergy for vehicle propulsion (e.g., the battery buffers fuel energy andrecovers kinematic energy in electric form). A PHEV is like a HEV, butthe PHEV may have a larger capacity battery that is rechargeable fromthe external electric grid. In a PHEV, the battery may be the mainsource of energy for vehicle propulsion until the battery depletes to alow energy level, at which time the PHEV operates like a HEV for vehiclepropulsion.

By way of example, FIG. 1 depicts a vehicle 10 as a battery electricvehicle (BEV) propelled by an electric motor 11 without assistance froman internal combustion engine. Motor 11 receives electrical power andprovides drive torque for vehicle propulsion. Motor 11 also functions asa generator for converting mechanical power into electrical powerthrough regenerative braking. Motor 11 is part of a powertrain 12 inwhich a gearbox 13 couples motor 11 to driven wheels 14. Gearbox 13adjusts the drive torque and speed of motor 11 by a predetermined gearratio.

Vehicle 10 includes a battery system 15 including a main battery pack 16and a battery energy controller module (BECM) 17. An output of batterypack 16 is connected to an inverter 18 which converts the direct current(DC) power supplied by the battery to alternating current (AC) power foroperating motor 11 in accordance with commands from a traction controlmodule (TCM) 20. TCM 20 monitors, among other things, the position,speed, and power consumption of motor 11 and provides output signalscorresponding to this information to other vehicle systems including amain vehicle controller 21 (which may be a powertrain control module, orPCM, for example).

FIG. 2 shows a typical bus architecture wherein a DC power source 30 isselectively coupled to a positive bus 31 and a negative bus 32 viacontactor switches 33. Buses 31 and 32 may be further coupled to aDC-to-DC converter 34, a link capacitor 35, and an inverter 36 whichdrives a traction motor 37. A chassis ground 40 represents conductiveparts of the vehicle whose electrical potential is taken as a referenceand which are conductively linked together.

Electrical isolation of buses 31 and 32 is determined by the electricalleakage resistance between each bus and chassis 40. A leakage resistance41 represents the level of isolation between positive bus 31 and chassis40. Leakage resistance 42 represents the isolation between negative bus32 and chassis 40. Leakage resistances 41 and 42 are the unbalanced ornonsymmetrical leakage resistances. While resistances 41 or 42 may occuronly one can be present for the unbalanced resistance. In addition, abalanced leakage resistance, 43 a and 43 b having the same resistance,may be present between buses 31 and 32 having a junction between themconnected to chassis 20. In addition, a balanced resistance may beintroduced across buses 31 and 32 within DC source 30, shown as leakageresistances 44 a and 44 b having their junction coupled to chassis 40.

FIG. 3 shows apparatus for detecting leakage resistance wherein a firstdetector circuit 45 is arranged between positive bus 31 and chassisground 40 and a second detector circuit 46 is arranged between negativebus 32 and chassis ground 40. First detector circuit 45 includes acurrent-limiting resistor 50 in series with a sampling switch 51 and acurrent-sensing resistor 52. A controller circuit 47 is connected toswitch 51 for selectively activating switch 51 so that a resulting firstcurrent flowing through detector circuit 45 creates a voltage acrosscurrent-sensing resistor 52 proportional to the current passing throughresistor 52 which is provided to controller circuit 47. Likewise, seconddetector circuit 46 includes a series connection of a current-limitingresistor 53, sampling switch 54, and current-sensing resistor 55similarly connected to controller circuit 47. Controller circuit 47 mayinclude a microcontroller such as in a battery energy controller module.

In the leakage resistance detecting system based on conventionalassumptions shown in FIG. 4, a current flowing through a currentlimiting resistor R_(d) and current sensing resistor R_(s) passesthrough leakage resistance R_(lm), which denotes leakage resistance tothe minus bus. Using a measured battery voltage V_(B), the firstmeasured current I_(rsm), and known resistance values for R_(d) andR_(s), leakage resistance R_(lm) is determined using the followingformula:

$\begin{matrix}{R_{l\; m} = {\frac{V_{B}}{I_{rsm}} - {\left( {R_{d} + R_{S}} \right).}}} & \left( {{Eq}.\mspace{14mu} 1} \right)\end{matrix}$

As previously explained, however, the resulting value for the leakageresistance does not take into account the balanced resistance whichinstead creates the more complex equivalent circuit shown in FIG. 5. Inthis case, the sense current is not entirely due to the parallelnegative bus leakage resistances R60 and R61 a. In addition, a currentflows through R61 b from the positive bus, providing an opposingcomponent of the balanced leakage current i_(bal). Hence, the totalsensed current under-represents the negative bus leakage current.

As seen from FIGS. 3 and 5, the total current through R_(lm) (theleakage resistance associated with negative bus 32) includes balancedpart from resistance 61 and a sense current I_(RS) (i.e., includingbalanced and unbalanced portions of resistances 44, 41, and 42 of FIG.3). When the sampling switch is closed, the current I_(RS) is

$\begin{matrix}{I_{RSM} = \frac{V_{B}R_{lp}}{{R_{C}R_{lp}} + {R_{l\; m}\left( {R_{C} + R_{lp}} \right)}}} & \left( {{Eq}.\mspace{14mu} 2} \right)\end{matrix}$

where R_(lp) is the leakage resistance 41 and balanced portion of 44associated with positive bus 31 (FIG. 3), R_(lm) is the leakageresistance 42 and balanced portion of 44 associated with negative bus 32(FIG. 3), I_(RSM) is the sense current I_(RS) as shown in FIG. 4 whenswitch 51 (FIG. 3) is closed, and resistance R_(C) is a combinedresistance (e.g., a fixed 519 k-ohms) which includes the sum of thecurrent-limiting resistance R_(D) and the current-sensing resistanceR_(S). While the following text assumes the values of R_(D) and R_(S) indetector circuits 45 and 46 are equal to each other, for those familiarwith the art these values may be changed while retaining the intent ofthis description. Thus, when the other sampling switch is closed, thecurrent I_(dp) can be determined using the formula:

$\begin{matrix}{I_{RSP} = {\frac{V_{B}R_{l\; m}}{{R_{C}R_{l\; m}} + {R_{lp}\left( {R_{C} + R_{l\; m}} \right)}}.}} & \left( {{Eq}.\mspace{14mu} 3} \right)\end{matrix}$

Equations 2 and 3 are solved simultaneously for leakage resistancesR_(lp) and R_(lm) as follows:

$\begin{matrix}{R_{l\; m} = {\frac{V_{B}}{I_{RSM}} - {R_{C}\left( {1 + \frac{I_{RSP}}{I_{RSM}}} \right)}}} & \left( {{Eq}.\mspace{14mu} 4} \right) \\{R_{lp} = {\frac{V_{B}}{I_{RSP}} - {R_{C}\left( {1 + \frac{I_{RSM}}{I_{RSP}}} \right)}}} & \left( {{Eq}.\mspace{14mu} 5} \right)\end{matrix}$

Thus, the ratios of the two measured currents provides respectivecorrection factors to the conventional determination of the respectiveleakage resistances. The resulting values for the positive and negativeleakage resistances, R_(lp) and R_(lm), based on the current ratiosprovide a more accurate assessment of the leakage resistances, whichconsequently enables more accurate determination of a correspondingelectrical isolation.

In a preferred embodiment, a bus having a lower isolation is used tocalculate an isolation value. Thus, the smaller of the calculatedleakage resistances is selected and then divided by a predeterminedvoltage in order to calculate the isolation value. The predeterminedvoltage can be comprised of the measured voltage (V_(B)) of the DCsource or a predetermined constant voltage (e.g., the nominal systemvoltage or a value specified by regulations). The isolation value can becalculated as follows:

$\begin{matrix}\frac{{Min}\left( {R_{lp},R_{l\; m}} \right)}{V_{B}} & \left( {{Eq}.\mspace{14mu} 6} \right)\end{matrix}$

The resulting isolation value is compared with an isolation threshold(e.g., 500 ohms/volt), and if it is less than the threshold then theinvention signals that an atypical condition has been detected. Thesignaling may be comprised of informing a driver of the condition and/orautomatically disconnecting the DC power source from the power buses(e.g., opening the contactor switches).

The calculated values for leakage resistances R_(lp) and R_(lm) canfurther be used to separate the balanced and unbalanced components ofthe resistances so that the components can be monitored over time inorder to detect or predict certain potential failures in the electricalsystem. Examples of balanced leakage resistances that can change overtime in a manner that identifies an impending fault include a)insulation breakdowns, and b) loss of effectiveness of a cooling-waterdeionizer in a fuel cell system. A plurality of leakage resistancemeasurements over time can be separated into balanced/unbalancedcomponents and stored in a database. Both the magnitude and slope withinthe stored data (e.g., either the balanced or unbalanced components) isused to predict potential failures. Examples of unbalanced leakageresistances that may change over time include contact of batteryterminals to chassis, wire contact to the chassis, and other forms ofcontact.

The calculated values for leakage resistances R_(lp) and R_(lm) can beseparated as follows. The larger one of leakage resistances R_(lp) andR_(lm) will correspond to the balanced resistance, i.e.,R_(bal)=max(R_(lp), R_(lm)). This is because, by definition, the valueof R_(bal) must be the same from chassis ground to both the positive andnegative buses. An unbalance resistance exists in parallel with thevalue of R_(bal), from either the positive or negative bus to ground.Since parallel resistances always result in a total resistance lowerthat each of the parallel resistances, the maximum of R_(lp) and R_(lm)corresponds to the balanced resistance.

The unbalanced resistance component can be calculated using R_(lp) andR_(m) as follows:

$\begin{matrix}{R_{unbal} = {\frac{R_{lp} \cdot R_{l\; m}}{R_{lp} - R_{l\; m}}}} & \left( {{Eq}.\mspace{14mu} 7} \right)\end{matrix}$

A method of the invention will be summarized in connection with theflowcharts of FIGS. 6 and 7. In step 70, the voltage V_(B) of the DCpower source is measured. In step 71, the positive bus is connected tothe chassis ground and a resulting current is measured. The negative busis then sequentially connected to chassis ground and a resulting currentis measured in step 72. In step 73, leakage resistances R_(LP) andR_(LM) are calculated. The smaller of the two resistances is chosen instep 74 in order to identify a worst-case bus leakage resistance. Instep 75, an isolation value is calculated using the chosen worst-caseresistance.

In step 76, the isolation value is compared to an isolation threshold.If less than the threshold, then the invention signals an atypicalcondition in step 77 (e.g., by notifying the driver or disconnectingpower to the high-voltage buses). If the isolation value is not lessthan the threshold, then the method is completed at step 78. Optionally,an additional method can be executed wherein the balanced and unbalancedresistance values are determined in order to provide prediction ofpotential failures as shown in FIG. 7.

In FIG. 7, the larger resistance value is assigned as the balancedleakage resistance in step 80. In step 81, an unbalanced leakageresistance is calculated (e.g., using Equation 7). The calculated valuesfor the balanced and/or unbalanced resistances are compiled over time instep 82. In step 83, predetermined potential failures can be predictedin response to predetermined decreases in the balanced or unbalancedresistances. More particularly, a potential failure can be predicted inresponse to a magnitude of a resistance value falling below a thresholdor in response to data points accumulated over time exhibiting anegative slope that points toward an impending drop in the resistancevalue below a threshold. For example, a predetermined decrease in thebalanced resistance component may correspond to a certain loss ineffectiveness in deionizing the coolant water for a fuel-cell (leadingto decreased power generation). Empirical testing may identify themagnitude or slope values of interest that can be used to detected aneed for corrective action to repair or replace the deionizer. Whenevera potential failure is detected or predicted, step 84 is executed toprovide notification to other vehicle systems (e.g., a controller fordeactivating the main contactors between the DC source and the buses),the driver (e.g., via a malfunction indicator light), or a remotemonitoring system (e.g., wirelessly to a manufacturer's database via anelectronic cloud).

What is claimed is:
 1. A method of monitoring bus leakage resistances inan electrified vehicle, comprising: measuring first and secondleakage-related currents for first and second buses by sequentiallyconnecting each bus to ground; estimating the bus leakage resistances inresponse to a bus voltage divided by the respective first and secondcurrents and in response to respective ratios of the first and secondcurrents; and checking a bus isolation value according to the estimatedbus leakage resistances.
 2. The method of claim 1 further comprising thesteps of: estimating a balanced leakage resistance by selecting a largerone of the estimated bus leakage resistances.
 3. The method of claim 2further comprising the step of: estimating an unbalanced leakageresistance according to a formula:$R_{unbal} = {\frac{R_{lp} \cdot R_{l\; m}}{R_{lp} - R_{l\; m}}}$where R_(lp) and R_(lm) are the estimated bus leakage resistances.
 4. Amethod of fault prediction in an electrified vehicle, comprising:measuring first and second leakage-related currents for first and secondbuses connected to a DC source by sequentially connecting each bus toground; estimating positive and negative bus leakage resistances inresponse to a bus voltage divided by the respective first and secondcurrents and in response to respective ratios of the first and secondcurrents; estimating a balanced leakage resistance by selecting a largerone of the estimated bus leakage resistances; and predicting a failurein response to predetermined decreases in the balanced leakageresistance over time.
 5. The method of claim 4 further comprising:estimating an unbalanced leakage resistance according to a formula:$R_{unbal} = {\frac{R_{lp} \cdot R_{l\; m}}{R_{lp} - R_{l\; m}}}$where R_(lp) and R_(lm) are the estimated bus leakage resistances; andpredicting a failure in response to predetermined decreases in theunbalanced leakage resistance over time.
 6. The method of claim 4wherein the step of predicting a failure is comprised of: comparing thebalanced leakage resistance to a predetermined threshold; and generatinga predicted failure notification in response to the balanced leakageresistance falling below the predetermined threshold.